Absorbent articles that include a wetness indicator are known. In some instances, the wetness indicator may include a colorant adapted to change in appearance, i.e., appear, disappear, change color, etc., upon contact with liquids such as, urine, runny bowel movements, menses, etc., in the article. Wetness indicators may be of the substantially insoluble type (i.e, they are designed to generally remains in the same location of the article before and after being wetted with liquid). Some wetness indicator compositions may be designed to simply disappear into the core of the diaper after being wetted (e.g., wetted with urine). Wetness indicator compositions typically function to serve at least one of the following: 1) the wetness indicator composition should effectively adhere, but not substantially bleed through, to the substrate on which it is applied (e.g., the backsheet) and should possess an optimum balance of cohesive strength and flexibility to remain intact during storage, as well as upon and after being wetted, 2) the initial color of the wetness indicator composition should not prematurely change color such that it confuses the caregiver or wearer as to whether a wetness event has occurred, 3) the color change of the wetness indicator composition should occur as quickly as possible after the wetness event, 4) the contrast in colors between the dry and wetted states of the wetness indicator composition should be great enough to signal the occurrence of the wetness event, 5) the color signaling the wetness event should remain visible for a long period of time after the wetness event and should not migrate to other regions of the diaper such that the signal denoting the wetness event becomes difficult or impossible to interpret, 6) the wetness indicators of the present invention should adhere, but not substantially bleed through, to the substrate that it comes in direct contact with (e.g., the nonwoven dusting layer) or that it is in close proximity to (e.g., the core cover) to provide for adequate wicking of fluid (e.g., urine), 7) the wetness indicator composition should remain stable (i.e., not change color prior to a wetness event) when placed in close proximity (if not direct contact) with higher (versus the colorant of the wetness indicator composition) pH absorbent article components, and 8) the wetness indicator composition should be easily made, easily processed for application onto the wearable article, be safe, and of economical cost.
While problems realized in the past (including high humidity and temperature environments) remain, there are additional challenges associated with new diaper designs, particularly including diaper designs comprising absorbent cores that are substantially cellulose free. These core designs comprise increased levels of absorbent polymer material (sometimes referred to as superabsorbent polymer or absorbent gelling material), adhesives (including, thermoplastic adhesive materials), and surfactants. Each of these comprise chemical compositions that can alter the pH of the wetness indicator composition and thus cause a color change prior to a wetness event. Particularly, alkaline surfactants containing amine, amide, or quaternary functionalities are especially problematic for wetness indicators comprising colorants that change color as the pH rises; but even lower pH moieties like carboxylates can also pose challenges and prematurely activate certain colorants within such wetness indicator compositions.
One color changing colorant active used in at least some wetness indicators is a pH indicator such as bromocresol green or the like, which changes color from yellow to blue in the pH range of 3.8 to 5.4. To maintain the yellow color of bromocresol green in a dry state, the wetness indicator composition may need to be relatively acidic. A wetness indicator that contains the bromocresol green pH indicator colorant typically remains yellow until it is contacted with a fluid such as urine. Upon contact with a liquid such as urine, the bromocresol green pH indicator colorant indicator will turn blue to indicate the presence of the liquid, due to the higher pH of the urine. In wetness indicator compositions that include a pH activated colorant, it may be desirable to incorporate acids (as well as other chemical components discussed in more detail below) in the wetness indicator composition to stabilize the wetness indicator (i.e., prevent an undesired color change of the pH indicators prior to a wetness event).
Placing the wetness indicator composition in close proximity to the absorbent core of a diaper, which is substantially cellulose free or includes a relatively high level of surfactant, may cause a premature color change in the colorant. This may be especially true when the diaper design utilizes lower basis weight and more breathable substrates (e.g., backsheets, dusting layers, and core covers) because these substrates may allow pH altering chemicals to come in closer proximity with the wetness indicator composition. Thus, it may be desirable to optimize the acid content of the wetness indicator composition relative to any other materials present in the composition to aid in preventing premature color change. But, while the acid content is optimized for stability, it may also be desirable to optimize the wetness indicator composition for proper functioning of the other properties (e.g., kinetics, dye retention, stability, adhesion). If too much acid or too strong of an acid mixture is incorporated, the pH can remain suppressed even after the wetness event such that the yellow color of the bromocresol green, for instance, persists and no blue color (signaling a wetness event) results even when a wetness event has occurred. If too little acid or too weak of an acid mixture is incorporated, the wetness indicator composition can prematurely change color. Thus, the acid content should be optimized so the wetness indicator composition remains stable under various storage scenarios, as well as within new absorbent article designs.
Beyond these stability issues, it may be desirable to configure the wetness indicator composition so that it contacts or even adheres to an inner adjacent layer of substrate (e.g., the dusting layer if the wetness indicator is disposed on the backsheet) to facilitate wicking of fluid (e.g., urine) to the wetness indicator composition after a wetness event occurs. That is, when a gap exists between the wetness indicator composition and the inner adjacent substrate (e.g., the dusting layer), fluid may not travel from the absorbent core to the wetness indicator within the desired time to signal a wetness event. It may additionally be desired that there is substantial contact/adherence of the wetness indicator to an inner adjacent substrate to ensure that the wetness indicator composition is evenly (top to bottom and side to side) wetted and thus provides the full signal intended. This can be accomplished by providing a wetness indicator composition that has an optimized “open time” during its incorporation into an absorbent article (e.g., the wetness indicator may be applied as a hot melt adhesive that is in a molten state long enough to adhere to the substrate, but not so long that it bleeds through the substrate or to other materials that could negatively impact the stability of the colorant). An ideal open time is long enough to allow efficient coating of the backsheet film but short enough so fast solidification of the wetness indicator prevents bleeding into regions that could negatively impact the stability of the colorant.
Accordingly, it would be desirable to provide a wetness indicator that is stable in the absorbent core of a disposable absorbent article. It would also be desirable to provide a wetness indicator that has a suitable open time for use in absorbent articles comprising lower basis weight and breathable films and nonwovens. It would further be desirable to provide a wetness indicator that has suitable color change kinetics.